how do I sand-cast porcelain?

Answer 1

This sand-casting method will only work with things that can have 1 side untouched.

Obytain the sand-casting sand from the hardware/arts supply store. Mix it and pour it into a container that is bigger than the shape you want to cast it into. Let the sand dry for aproximatly 3 hours, or until moist to touch but will stand up on it's own. Carve out the negative of the final shape that you want into the sand while it is still moist (note: smooth the clay very carefully and very smooth or the texture of the porcelain will be terriable). Let this set for 2 to 6 hours. Pour in a slurry of the porcelain clay. Shake it lightly to let any air bubbles come to the untouched surface (note: be careful not shake too hard as this removes the details that you want in the cast.). Let the porcelain dry until it is ready to fire. Loosen and remove some sand in a line about 2 inches around the clay, leaving sand around the clay alone. Place a thick foam mat on top of the cast, and slowly flip the container over. Brush away/remove the sand around the porcelain and brush away any excess particles. Fire as usual.

Answer 2

I would say that it is a useless operation to attempt. Porcelain toilet fixtures are cast from slip in plaster of paris molds which suck the moisture out and leave a clay surface.
I have done thin clay (not porcelain) and shaped it onto a sculpted packed sand surface to make clay forms for sagging glass, letting the clay get leather hard before moving it, but of course, the sand leaves a texture on one side that is far from the one expected on porcelain - it might be better with finer sand than I use.
http://users.ticnet.com/mikefirth/warmglas.htm#MOLDOVLHRT
An attempt to pour porcelain into sand castings would be fraught with all kinds of risks and why would it thicken?